Over the course of building and maintaining an elementary school computer lab for the
last six years, I've encountered just about every pre-PCI Mac, either
through buying cheap ones at swap meets or having them donated.
Inevitably some of these machines had bad power supplies or bad logic
boards. Some were kept for their working floppies, and some had custom
huge hard disks, while others were little more than a working logic
board in a busted case.
Having all these parts machines lying about, I began to notice
similarities in power supplies and logic board arrangement between
otherwise completely different computers. The first time I noticed this
was when I had a Quadra 700 and a
Quadra 650 torn apart on the same
table in the back of my room ("the shop"). The logic boards looked very
similar. They had the securing holes in the same places, the power
supplies connected at the same spot, even the ports (SCSI, ADB, serial,
AAUI) on the back all lined up. Since I already had them apart, as an
experiment I tried the 700 logic board in the 650 case. Lo and behold!
Ding! Happy Mac.
This led me to compare other logic boards, which in turn gave me
some insights into the evolution of the pre-PCI Macs. For instance: the
IIcx sized logic board lasted
seven-and-a-half years (3/89 - 9/96) through four different case
changes. Obviously the Q700 logic boards were the same size as IIcx's,
and they used the same power supply, but logic boards that fit the
IIvi case (Q650, PM 7100) were the same size and
shape, too. So were those in the Q800 (PM 8100) case.
So if you have a bad logic board in, say, a Q800, and you also have
a working IIci, you could make a working computer with room for a built
in CD-ROM drive out of the two. You could consider this a case upgrade
for the IIci or a logic board downgrade for the Q800. You would have to
do some dremmel work on the case's back panel to get to some of the
ports, but it would work (at least till you could find a 8100 board to
put in there).
Below is a table of pre-PCI Mac logic board "form factors." Please
be careful if you try to do some of these swaps. Just because the logic
board is the same shape, don't assume the power supplies are the same
too. For example: even though the Performa 6360 uses an identical looking
outer case, you can't put its logic board in a Quadra 630 case, because the 6360 board
requires a heftier power supply. The 630 board might work in the 6360
case, though.
A similar case may not be a guarantee the logic boards will even
fit. The LC 575 and LC 580 have an almost identical case
design, but the LC 580 logic board is actually a member of the IDE
friendly Q630 family, not the SCSI only Color Classic sized boards
of the other 5xx's.
In the table, the main model names (Mac II, Centris, Quadra, LC,
PowerMac) are used, but there may be Performa variations not
mentioned.
Something struck me after I made the chart. The Macintosh IIsi
design was never used again for anything else. Its power supply, case,
and logic board are unique among Macs. Was there ever another desktop
Mac design that was only used once, without even any upgrades?
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